FREQUENTLY ASKED QUESTIONS - SUMMARY

Most weather radars, such as the National Weather Service NEXRAD radar,
transmit radio wave pulses that have a horizontal orientation. Polarimetric
radars (also referred to as dual-polarization radars), transmit
radio wave pulses that have both horizontal and vertical orientations.
The horizontal pulses essentially give a measure of the horizontal dimension
of cloud (cloud water and cloud ice) and precipitation (snow, ice pellets,
hail, and rain) particles while the vertical pulses essentially give a
measure of the vertical dimension. Since the power returned to the radar
is a complicated function of each particles size, shape, and ice density,
this additional information results in improved estimates of rain and snow
rates, better detection of large hail location in summer storms, and improved
identification of rain/snow transition regions in winter storms.

Engineers at the National Severe Storms Laboratory are currently working
to develop a polarimetric NEXRAD Doppler radar. Polarimetric technology
should be available for installation into the national radar network in
5-10 years.

How do weather radars work?

The word radar is actually an acronym for radio detection and
ranging. Therefore, as implied by the name, energy transmitted by weather
radars is essentially a radio wave with a frequency at the high end of
the radio spectrum. That is, if you could tune your radio up the frequency
dial to a location well beyond the upper limit where radio stations broadcast,
you would eventually enter the frequency regime used by weather radars.
Corresponding to any radio wave frequency is a uniquely defined distance
from one wave crest to the next, referred to as the wavelength. For NEXRAD,
the frequency and wavelength are typically around 3000 Megahertz and 10
cm (4 inches), respectively.

Weather radars transmit short pulses of radio waves at a rate
of approximately 1000 pulses per second, with each pulse lasting only about
a millionth of a second. After each pulse, there is a short
time period during which the radar is not transmitting. Rather, during
this time the radar is listening for a "reflected signal" from the cloud.
This reflected signal is the result of the energy from the transmitted
pulse interacting with cloud (cloud water and cloud ice) and precipitation
(snow, ice pellets, hail, and rain) particles. A small portion of the power
is then returned to the radar, received by the antenna, and analyzed by
the radar signal processor to determine an estimate of the rain or snow
rate.

Doppler
radars have the added capability of being able to measure a frequency
shift that is introduced into the reflected signal by the motion of
the cloud and precipitation particles. This frequency shift is then used
to determine wind speed.

Here, we discuss topics that will hopefully lead to a better understanding
of polarimetric radar design, operation, and capabilities.

How do cloud and precipitation particles interact with
radio waves?

The radio wave frequency used by weather radars is specifically chosen
for its ability to "interact" with cloud and precipitation particles. Radio
waves with lower frequencies, such as those transmitted by radio stations,
tend to pass through clouds. Therefore, they are not well suited for weather
radar applications.

Other than the Doppler frequency shift, there are essentially two processes
by which the cloud and precipitation particles interact with the radar's
power and phase (position of the wave crest). The first is backscattering.
The second is a propagation effect.

Backscattering refers to the process whereby each individual cloud and
precipitation particle intercepts the radio wave and essentially reflects
a portion of the power back towards the radar. The total amount of power
that is reflected is a complicated function of the size, shape, and ice
density of each cloud and precipitation particle. The importance of these
parameters are discussed in more detail in the next section.

Propagation effect refers to the process whereby cloud and precipitation
particles, as a whole, work to modify the power and phase of the transmitted
signal. These changes gradually accumulate as the wave propagates though
the atmosphere, clouds, and precipitation (and, after being reflected,
back through the precipitation, clouds, and atmosphere to the radar again).
During this two way trip, some of the radar's energy is absorbed resulting
in a decrease in power known as attenuation. There is also frequently a
slight shift in phase of the wave, especially in regions of heavy rain.

Together, an understanding of backscattering and propagation effects is
essential to interpret polarimetric radar signatures.

What causes uncertainties in radar precipitation estimates?

As noted earlier, the reflected power returned to the radar is related
to the size, shape, and ice density of each cloud and precipitation particle
that it illuminates. What do we mean by illuminate? Well, think of the
radar as a big flash light. If you are standing in a dark room with a flashlight,
the closer you are to a wall, the brighter the beam is and the smaller
the area that it illuminates. However, as you back away from the wall,
the weaker the returned power and the larger the area that is illuminated.
Such is the case with radars. As you get further from the cloud, the width
of the radar beam broadens, the power becomes more diffuse, and the number
of cloud and precipitation particles "illuminated" by the radar becomes
larger.

Given the wide variety of precipitation types, shapes, and sizes of
cloud and precipitation particles, it is easy to see how the interpretation
of reflected radar signal can become quite complicated. Listed below are
just a few of the complicating factors:

The relationship between the size and power return is highly non-linear.
As an example, lets use the case of a very small, spherical drop. If you
double the size of a the drop, you increase the reflected power return
by a factor of 64. If you triple the size of the drop, you increase the
reflected power return by a factor of 729. And so on. Of course, this problem
is even more complicated when you consider that only the very smallest
of drops are spherical.

Clouds and precipitation frequently consist of a variety of particle types.
For example, rain and hail, rain and snow, and snow and ice pellet mixtures
are all quite common. Depending on its size, shape, and ice density, each
cloud and precipitation particle interacts with the radar's energy in its
own unique way.

At a distance of 60 km from the radar, the horizontal and vertical width
of the beam of energy is almost 1 km. The wider the beam, the greater the
likelihood of sampling a mixture of precipitation types, especially in
the vertical considering that ice particles melt and change shape as they
fall.

Natural obstacles (such as hills and trees) and man made obstacles (such
as buildings and power poles) frequently block a portion of the radar beam,
resulting in an artificially high power return.

Combined, these problems sometimes make radar precipitation estimation
very difficult. Using a new pulse transmission scheme, polarimetric radars
are designed to eliminate many of these problems.

What is meant by polarization?

Technically speaking, a radio wave is a series of oscillating electric
and magnetic fields. Of course, you can't actually see the oscillating
electric and magnetic fields, but the radar can detect and interpret them
just as your car radio can detect and interpret those transmitted at the
slightly lower frequencies. If, however, we could see them, they would
look something like the waves depicted below in Fig. 1.

Figure 1a: Example of the structure of a horizontally polarized
radio wave. The electric field wave crest is oriented in the horizontal
direction (blue in this figure). The magnetic field wave crest is oriented
in the vertical direction (white in this figure).

Figure 1b: Example of the structure of a vertically polarized
radio wave. The electric field wave crest is oriented in the vertical direction
(red in this figure). The magnetic field wave crest is oriented in the
horizontal direction (white in this figure).

As can be seen in Fig. 1, the electric and magnetic fields are oriented
at 90 degree angles to each other. This concept is important for understanding
what is meant by polarization. That is, the polarization of the
radio wave is defined as the direction of orientation of the electric
field wave crest. Thus, in Fig 1a, the polarization is horizontal since
the electric field wave crest (shown in blue) is aligned along the horizontal
axis. In Fig. 1b, the polarization is vertical since the electric field
wave crest (shown in red) is aligned along the vertical axis.

Polarimetric radars gain additional information about the precipitation
characteristics of clouds by essentially controlling the polarization of
the energy that is transmitted and received.

What is a polarimetric radar?

Most weather radars, including NEXRAD, transmit and receive radio waves
with a single, horizontal polarization. That is, the direction of the electric
field wave crest is aligned along the horizontal axis. Polarimetric radars,
on the other hand, transmit and receive both horizontal and vertical
polarizations. Although there are many different ways to mix the horizontal
and vertical pulses together into a transmission scheme, the most common
method is to alternate between horizontal and vertical polarizations with
each successive pulse. That is, first horizontal, then vertical, then horizontal,
then vertical, etc. And, of course, after each transmitted pulse there
is a short listening period during which the radar receives and interprets
reflected signals from the cloud.

Since polarimetric radars transmit and receive two polarizations of
radio waves, they are sometimes referred to as dual-polarization
radars. The difference between non-polarimetric and polarimetric radars
is illustrated below:

NEXRAD (non-polarimetric) Radar

Figure 2a: Non-polarimetric radars, such as NEXRAD, transmit
and receive only horizontal
polarization radio wave pulses. Therefore, they measure only the horizontal dimension
of cloud and precipitation particles.

Polarimetric Radar

Figure 2b: Polarimetric radars transmit and receive both horizontalandvertical
polarization radio wave pulses. Therefore, they measure both the horizontal and
vertical dimensions of cloud and precipitation particles. This additional
information leads to improved radar estimation of precipitation type and
rate.

Engineers at the National Severe Storms Laboratory are currently working
to develop a polarimetric NEXRAD Doppler radar. Polarimetric technology
should be available for installation into the national radar network in
5-10 years. It should be noted that a polarimetric upgrade does not replace
Doppler technology. Rather, it complements it. Doppler capabilities give
information on cloud motion. Polarimetric capabilities give information
on precipitation type and rate.

What does a polarimetric radar measure?

All weather radars, including NEXRAD, measure horizontal reflectivity.
That is, they measure the reflected power returned from the radar's horizontal
pulses. Polarimetric radars, on the other hand, measure the reflected power
returned from both horizontal and vertical pulses. By comparing these reflected
power returns in different ways (ratios, correlations, etc.), we are able
to obtain information on the size, shape, and ice density of cloud and
precipitation particles.

Some of the fundamental variables measured by polarimetric radars, and
a short description of each, are listed below:

Differential Reflectivity - The differential reflectivity is a ratio of
the reflected horizontal and vertical power returns. Amongst other things,
it is a good indicator of drop shape. In turn, the shape is a good estimate
of average drop size.

Correlation Coefficient - The correlation coefficient is a correlation
between the reflected horizontal and vertical power returns. It is a good
indicator of regions where there is a mixture of precipitation types, such
as rain and snow.

Linear Depolarization Ratio - The linear depolarization ratio is a ratio
of a vertical power return from a horizontal pulse or a horizontal power
return from a vertical pulse. It too is a good indicator of regions where
a mixture of precipitation types occur.

Specific Differential Phase - The specific differential phase is a comparison
of the returned phase difference between the horizontal and vertical pulses.
This phase difference is caused by the difference in the number of wave cycles
(or wavelengths) along the propagation path for horizontal and vertically
polarized waves. It should
not to be confused with the Doppler fequency shift, which is caused by
the motion of the cloud and precipitation particles. Unlike the differential
reflectivity, correlation coeffiecient, and linear depolarization ratio,
which are all dependent on reflected power, the specific differential phase
is a "propagation effect". It is a very good estimator of rain rate.

While other variables that are commonly referred to as "cross-polar terms"
are also measured, they are largely unexplored. Indeed, there is yet much
to be learned about polarimetric radar signals.

Can you give a simple example of an improvement offered
by polarimetric radar?

Since the power returned to the radar is such a complicated function of
the size, shape, and ice density of each cloud and precipitation particle,
any information that we can gain about the average characteristics
of the precipitation help us to better determine rain rates, snow rates,
or even possibly the size of hail.

Rainfall Example

As a simple example of how a polarimetric radar can give additional
information on precipitation type and rate, let us examine a hypothetical
rain event. When compared to snow, for example, rain is a very simple precipitation
type. Well, suppose it were raining and you somehow had the magical ability
to suddenly stop the rain from falling, go outside to grab a cubic meter
of air (including the suspended rain drops), and then take it all inside
for examination. Once inside, you start removing the individual rain drops,
examining each of their sizes, and adding up the total water content to
get an estimate of the rain rate. For the sake of this discussion, lets
assume that the first time you do this you find a few very big drops, no
small ones, and get a rain rate of 0.5 inch per hour. You wait 15 minutes
and then repeat the experiment. But this time you find a large number of
very small drops and no big ones. But, much to your surprise, you again
get a rain rate of 0.5 inch per hour!

How can this be? The number and average size of the rain drops has changed
dramatically but the rain rate has not. It is because, in the first sample,
the rain water was concentrated in a very small number of large drops
and, in the second sample, the rainwater was concentrated in a very
large number of small drops. Yet, since the reflected power returned
to the radar is heavily weighted towards the largest drops (discussed in
an earlier section), the power returned to the radar from the first sample
might be as much as 10 times greater than the power returned to the radar
from the second sample! As you can see, if you are strictly using the returned
power to estimate rain rate, you might end up with either a significant
overestimation or a significant underestimation of the rain rate. It would
all depend on the dominant drop size. This can be a severe limitation of
non-polarimetric radars.

Now suppose we have a polarimetric radar. So far, we have been assuming
that rain drops are spherical in shape. In reality, that is only true for
the very smallest drops. For bigger drops, drag forces as they fall through
the atmosphere causes a flattening effect that results in an almost "hamburger
bun" type appearance for the very big drops. If we had a polarimetric radar,
we would measure differential reflectivity. That is,
we would first transmit and receive a horizontal pulse of energy. This
would give us an indication of the horizontal dimension of the drop. We
would then transmit and receive a vertical pulse of energy. This would
give us an indication of the vertical dimension of the drop. Combined,
we could use this information to get a measure of the average drop shape
and, in turn, dominant drop size. This could be used to refine the radar
rain rate estimate.

Of course, understanding polarimetric radar power returned from oddly
shaped snow, ice crystals, hail, and regions that contain mixtures of precipitation
types can get quite complicated. But in each case, polarimetric radars
allow us to obtain more information on the overall precipitation structure
of the cloud.

How can polarimetric radar measurements lead to better
weather predictions?

Radars won't tell you if it is going to rain tomorrow. However, once a
cloud does develop and precipitation starts falling, they can be used to
examine storm structure and estimate rain and snow rates.

The improvements associated with polarimetric radars comes from their
ability to provide previously unavailable information on cloud and precipitation
particle size, shape, and ice density. With this in mind, just a few of
the potential applications of polarimetric radar data are listed below.

Improved estimation of rain and snow rates.

Discrimination of hail from rain and possibly gauging hail size.

Identification of precipitation type in winter storms.

Identification of electrically active storms.

Identification of aircraft icing conditions.

Techniques are also being developed to use mathematical functions to weight
the relative importance of the polarimetric variables as they relate to
identifying each cloud (cloud water and cloud ice) and precipitation (snow,
ice pellets, hail, and rain) particle type. For example, differential reflectivity
may do a better job identifying one particle type, whereas specific differential
phase may do a better job identifying another. By combining the weights
for each variable, a "classification" of the dominant particle type can
be determined for each portion of the cloud. This information can be used
to improve predictions from short-term computer forecast models.

What else can a polarimetric radar detect?

In addition to providing information on cloud and precipitation particle
size, shape, and ice density, polarimetric radar variables also exhibit
unique signatures for many non-meteorological scatterers. Examples would
be birds and insects. Though radar measurements of birds and insects may
not at first appear to be of interest to meteorologists, there are indeed
applications. For example, the motion of the brids and insects may affect
the measured Doppler winds, occasionally making interpretation of wind
fields difficult. Radar measurements of birds and insects may also be of
interest to other commercial and scientific disciplines. For example, birds
are hazardous to aircraft. Therefore, radar measurements of birds might
interest the aviation industry. Radar measurements of bugs might interest
entomologists who study, for example, crop damage resulting from bug migrations.
For weather radars, of course, birds and bugs are generally thought of
as a data contamination. Fortunately, their unique polarimetric signatures
generally make them easily identifiable in the data. This is not always
the case for non-polarimetric radars.

Another problem that frequently plagues radar measurements is the presence
of anomalous propagation (commonly referred to as AP). AP refers to a "ground
return contamination" that sometimes occurs in the radar data when a warm
layer of air forms above a cold layer of air. This phenomena, which is
called an inversion layer, essentially bends the radar beam back towards
the ground resulting in a ground return contamination that makes it very
difficult, at times, to distinguish the location and intensity of clouds
and precipitation. Polarimetric radar signatures also aid in the elimination
of AP.